Problem: Which of these numbers is prime? ${15,\ 19,\ 74,\ 75,\ 87}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 15 are 1, 3, 5, and 15. The factors of 19 are 1 and 19. The factors of 74 are 1, 2, 37, and 74. The factors of 75 are 1, 3, 5, 15, 25, and 75. The factors of 87 are 1, 3, 29, and 87. Thus, 19 is a prime number.